Neuromodulation of crossmodal influences on visual cortex excitability

Crossmodal interactions occur not only within brain regions deemed to be heteromodal, but also within primary sensory areas, traditionally considered as modality-specific. So far, mechanisms of crossmodal interactions in primary visual areas remain largely unknown. In the present study, we explored the effect of crossmodal stimuli on phosphene perception, induced by single-pulse transcranial magnetic stimulation (sTMS) delivered to the occipital visual cortex. In three experiments, we showed that redundant auditory and/or tactile information facilitated the detection of phosphenes induced by occipital sTMS, applied at sub-threshold intensity, which also increased their level of brightness, with the maximal enhancement occurring for trimodal stimulus combinations. Such crossmodal enhancement can be further boosted by the brain polarization of heteromodal areas mediating crossmodal links in spatial attention. Specifically, anodal transcranial direct current stimulation (tDCS) of both the occipital and the parietal cortices facilitated phosphene detection under unimodal conditions, whereas anodal tDCS of the parietal and temporal cortices enhanced phosphene detection selectively under crossmodal conditions, when auditory or tactile stimuli were combined with occipital sTMS. Overall, crossmodal interactions can enhance neural excitability within low-level visual areas, and tDCS can be used for boosting such crossmodal influences on visual responses, likely affecting mechanisms of crossmodal spatial attention involving feedback modulation from heteromodal areas on sensory-specific cortices. TDCS can effectively facilitate the integration of multisensory signals originating from the external world, hence improving visual perception.